1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a CMOS image sensor and a method for fabricating the same, which improves the sensor's image characteristics by employing an additional structural layer for closing a gap between elements of a color filter array.
2. Discussion of the Related Art
Image sensors are semiconductor devices that convert an optical image into an electrical signal and include charge-coupled devices and complementary metal-oxide-semiconductor (CMOS) image sensors. A typical charge-coupled device includes an array of photodiodes converting light signals into electrical signals. This set up, however, has a complicated driving method, high power consumption, and a complicated fabrication process requiring a multi-phased photolithography process. Additionally, in a charge-coupled device, the integration of complementary circuitry such as a control circuit, a signal processor, and an analog-to-digital converter into a single-chip device is difficult, thereby hindering development of compact-sized (thin) products, such as digital still cameras and digital video cameras, that use such image sensors.
CMOS image sensors, on the other hand, adopt CMOS technology that uses a control circuit and a signal processing circuit as a peripheral circuit and adopts switching technology which allows outputs to be sequentially detected using MOS transistors provided in correspondence with the number of arrayed pixels. Additionally, a CMOS image sensor uses CMOS fabrication technology, which is a simple fabrication method with fewer photolithography steps, that enables an advantageous device exhibiting low power consumption.
Typically, in the aforementioned CMOS image sensor, a photodiode is the active device that forms an optical image based on incident light signals by generating electrical signals according to the intensity and wavelength (color) of the incident light. In such a CMOS image sensor, wherein each photodiode senses incident light and a corresponding CMOS logic circuit converts the sensed light into an electrical signal according to the input wavelength, the photodiode's photosensitivity increases as more light is able to reach the photodiode. One way of enhancing a CMOS image sensor's photosensitivity is to improve its “fill factor,” i.e., the degree of surface area covered by the photodiodes versus the entire surface area of the image sensor. The fill factor is improved by increasing the area responsive to incident light. The concentration of incident light onto the photodiode is further facilitated when the quantum efficiency at all wavelengths (white light) is “1,” which represents a balanced transmission to the photodiodes across the spectrum to include complimentary components of red light, blue light, and green light received by the photodiodes.
To redirect any light that may be incident to the image sensor outside the immediate area of the photodiodes and to concentrate (focus) the incident light on one or more of the photodiodes themselves, a device of a material exhibiting excellent light transmittance, such as a convex microlens having a predetermined curvature for refracting incident light, may be provided. Incident light striking the surface of the convex structure of the microlens while in parallel to the optical axis of the microlens is refracted by the microlens according to the curvature of the convex microlens and is thereby focused at a predetermined point along the optical axis. As FIG. 1 illustrates, a CMOS color image sensor according to the related art may be provided with a microlens layer over a color filter layer including red (R), blue (B), and green (G) filter elements for focusing the light of each color (wavelength).
Referring to FIG. 1, a CMOS image sensor according to the related art includes a semiconductor substrate 100, on which a plurality (array) of photodiodes 110 are arranged at fixed intervals; a light-shielding layer 120 for allowing a light signal reception at each photodiode only by blocking any light arriving between the photodiodes; an insulating interlayer 130 formed over the structure of the light-shielding layer and photodiode array to receive a color filter layer 150 which is patterned into a color filter array including R, G, and B elements corresponding to the photodiode array; and a planarization layer 160 formed over the entire resulting structure to receive a plurality of microlenses 170 corresponding to the color filter array. The individual color filters of the color filter layer 150, which are arranged at fixed intervals, are formed by coating and patterning a corresponding photosensitive material on the insulating interlayer 130. Each coating is patterned by a photolithography step using a separate mask to form a corresponding array of pixels. Each element of the color filter layer 150 is arranged to transmit light of a predetermined wavelength corresponding to the element color.
A predetermined gap inherently occurs along the junctures between adjacently arranged filters that are meant to filter different colors. While the size of this gap is preferably kept to a minimum, its existence is due to the unavoidable error tolerances of the photolithography process used to form the color filter layer during the patterning process of the respective colored resists that make up the color filter layer. The presence of the gap, which is especially problematic at the junction of four adjacently disposed filters arranged as part of the Bayer pattern of a color filter array, precludes optimum photodiode operation because the incident light allowed to pass unfiltered by the color filter layer degrades the image characteristics.